Xtreme

I was thinking what if we could keep advancing and maybe even go past the 21st century. I know it'll be difficult and complicated but I was thinking something along the lines of empire earth. There could be underground resources such as oil etc. when surface resources run out in the later ages. A tree plantation to grow more trees? Open cut mines? What do you think? Would it be hard to implement firearms? This could be a long term goal of 0 ad?